Parse basic blocks and instructions.

Create map entries for ebbs and values as they are defined, but leave ebb and
value operands unresolved on instructions as they are parsed. Instruction
operands can refer to ebbs and values that may not have been defined yet.

Don't infer or check result types yet.

src/libcretonne/repr.rs

@@ -19,6 +19,9 @@ pub struct Function {
     /// Signature of this function.
     signature: Signature,
 
+    /// The entry block.
+    pub entry_block: Ebb,
+
     /// Stack slots allocated in this function.
     stack_slots: Vec<StackSlotData>,
 
@@ -45,6 +48,7 @@ impl Function {
         Function {
             name: name,
             signature: sig,
+            entry_block: NO_EBB,
             stack_slots: Vec::new(),
             instructions: Vec::new(),
             extended_basic_blocks: Vec::new(),